Kinetics of reversible reductive carbonylation of heme in human cystathionine ß-synthase.
Biochemistry
; 52(26): 4553-62, 2013 Jul 02.
Article
em En
| MEDLINE
| ID: mdl-23790103
Cystathionine ß-synthase (CBS) catalyzes the condensation of homocysteine with serine or cysteine to form cystathionine and water or hydrogen sulfide (H2S), respectively. In addition to pyridoxal phosphate, human CBS has a heme cofactor with cysteine and histidine as ligands. While Fe(III)-CBS is inert to exogenous ligands, Fe(II)-CBS can be reversibly inhibited by carbon monoxide (CO) and reoxidized by O2 to yield superoxide radical. In this study, we have examined the kinetics of Fe(II)CO-CBS formation and reoxidation. Reduction of Fe(III)-CBS by dithionite showed a square root dependence on concentration, indicating that the reductant species was the sulfur dioxide radical anion (SO2(â¢-)) that exists in rapid equilibrium with S2O4(2-). Formation of Fe(II)CO-CBS from Fe(II)-CBS and 1 mM CO occurred with a rate constant of (3.1 ± 0.4) × 10(-3) s(-1) (pH 7.4, 25 °C). The reaction of Fe(III)-CBS with the reduced form of the flavoprotein methionine synthase reductase in the presence of CO and NADPH resulted in its reduction and carbonylation to form Fe(II)CO-CBS. Fe(II)-CBS was formed as an intermediate with a rate constant of (9.3 ± 2.5) × 10(2) M(-1) s(-1). Reoxidation of Fe(II)CO-CBS by O2 was multiphasic. The major phase showed a hyperbolic dependence on O2 concentration. Although H2S is a product of the CBS reaction and a potential heme ligand, we did not find evidence of an effect of exogenous H2S on activity or heme binding. Reversible reduction of CBS by a physiologically relevant oxidoreductase is consistent with a regulatory role for the heme and could constitute a mechanism for cross talk among the CO, H2S, and superoxide signaling pathways.
Texto completo:
1
Coleções:
01-internacional
Base de dados:
MEDLINE
Assunto principal:
Oxigênio
/
Monóxido de Carbono
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Cistationina beta-Sintase
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Heme
Limite:
Humans
Idioma:
En
Revista:
Biochemistry
Ano de publicação:
2013
Tipo de documento:
Article
País de afiliação:
Uruguai
País de publicação:
Estados Unidos